CRITICAL DIMENSION UNIFORMITY OPTIMIZATION
Embodiments of an apparatus and methods for providing critical dimensions of a pattern. Pattern parameters and process history from a first substrate are used to create a thermal modes. The thermal mode is employed to established intelligent set points for zones of a substrate heater. A second substrate is position proximate the heater. The actual temperature of each zone is controlled using the corresponding intelligent setpoint.
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The field of invention relates generally to the fields of semiconductor device and microelectromechanical system manufacturing and, more specifically but not exclusively, relates to the optimization of critical dimension uniformity.
BACKGROUND INFORMATIONCircuit feature patterns used in the manufacture of a semiconductor device, a liquid crystal display (LCD) or a microelectromechanical device are partially derived from a series of deposition, photolithography, etching, and cleaning processes on a wafer or substrate. Process information may be gathered and stored for each wafer processed, identifying the process history of each wafer. The process history may contain process recipe, tool and chamber identification, a time history, in-line parametric data, or defectivity maps, or other information specific to the manufacturing steps used to create the semiconductor device, LCD or microelectromechanical device. The process history can include information for all or any portion of the processes to which each wafer is subjected. The process history may be stored for a given time to allow, for example, production and technical personnel to identify sources of variability or other problems in the manufacturing process. Process tools and chambers may be scheduled in a feed-forward manner to provide a wafer or substrate a planned path for processing depending on the specific design requirements of the circuit feature.
Post exposure bake (PEB), a heat-treating sub-process in the photolithography process, may play a role in establishing circuit feature characteristics. Thermally treating a resist with a hotplate in a thermal or coating developing system may have many purposes, from removing a solvent to activating a chemically amplified resist (CAR).
Chemically amplified resists were developed because of the low spectral energy of DUV radiation. A CAR comprises one or more components, such as chemical protectors, that are insoluble in the developer and other components, such as a photoacid generator (PAG). During an exposure step, the PAGs produce acid molecules that include the image information. The acid molecules may remain inactive until a PEB is performed. The PEB drives a deprotection reaction forward in which the thermal energy causes the acid to react with the chemical protectors.
The present invention is illustrated by way of example and not as a limitation in the figures of the accompanying drawings, in which
An apparatus and methods for providing critical dimensions through heat treatment is disclosed in various embodiments. However, one skilled in the relevant art will recognize that the various embodiments may be practiced without one or more of the specific details, or with other replacement and/or additional methods, materials, or components. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention. Similarly, for purposes of explanation, specific numbers, materials, and configurations are set forth in order to provide a thorough understanding of the invention. Nevertheless, the invention may be practiced without specific details. Furthermore, it is understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, but do not denote that they are present in every embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. Various additional layers and/or structures may be included and/or described features may be omitted in other embodiments.
Various operations will be described as multiple discrete operations in turn, in a manner that is most helpful in understanding the invention. However, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. Operations described may be performed in a different order than the described embodiment. Various additional operations may be performed and/or described operations may be omitted in additional embodiments.
Beginning with the illustration in
According to the embodiment illustrated in
The cooling unit (COL) 135 and the extension cooling unit (EXTCOL) 170 may be operated at low processing temperatures and arranged at lower stages, and the prebaking unit (PREBAKE) 155, the postbaking unit (POBAKE) 160 and the adhesion unit (AD) 145 are operated at high temperatures and arranged at the upper stages. With this arrangement, thermal interference between units may be reduced. Alternatively, these units may have different arrangements. The prebaking unit (PREBAKE) 155, the postbaking unit (POBAKE) 160, and the adhesion unit (AD) 145 each comprise a heat treatment apparatus in which substrates are heated to temperatures above room temperature.
As illustrated in
A plurality of projections 230 may be located on an upper surface of the hotplate 205 for accurately positioning the substrate 390. In addition, a plurality of smaller projections (not shown) may be formed on the upper surface of the hotplate 205. When the substrate 390 is mounted on the hotplate 205, top portions of these smaller projections contacts the substrate 390, which produces a small gap between the substrate 390 and the hotplate 205, thereby preventing the lower surface of the substrate 390 from being strained and damaged.
The ring-form shutter 225 is positioned at a place lower than the hotplate 205 at non-operation time, whereas, at an operation time, the ring-form shutter 225 is lifted up to a position higher than the hotplate 205 and between the hotplate 205 and a cover (not shown). When the ring-form shutter 225 is lifted up, a cooling gas, such as nitrogen gas or air, is exhausted from air holes below the hotplate 205.
As illustrated in
Hotplate 205 may have a circular shape and may comprise a number of segments. In addition, heater 325 may comprise a number of heating elements. For example, a heating element may be positioned within each segment of the hotplate 205. In an alternate embodiment, hotplate 205 may incorporate a cooling element and/or a combined heating/cooling element rather than a heating element.
Hotplate 205 may include a sensor 330, which may be a physical sensor and/or a virtual sensor. In addition, sensor 330 may comprise a number of sensor elements. For example, sensor 330 may be a temperature sensor located within each hotplate segment. In addition, sensor 330 may include at least one pressure sensor. Controller 310 is coupled to heater 325 and sensor 330. Various types of physical temperature sensors 330 may be used. For example, the sensor 330 can include a thermocouple, a temperature-indicating resistor, a radiation type temperature sensor, and the like. Other physical sensors 330 include contact-type sensors and non-contact sensors.
Heat treatment apparatus 300 may be coupled to a processing system controller 380 that is capable of transferring pattern parameter data, process history, and process flow information to and from the heat treatment apparatus 300. The pattern parameter data, process history and process flow information may be received and/or transmitted to another system by the processing system controller 380 through one or more ports. In one embodiment, a port is a wired communications pathway such as SEMI Equipment Communications Standard/Generic Equipment Model (SECS/GEM) interface. In another embodiment, the port is a wired Ethernet connection. Pattern parameter data may include optical digital profile (ODP) data, such as critical dimension (CD) data, profile data, or uniformity data, or optical data, such as refractive index (n) data or extinction coefficient (k) data. For example, CD data measurements collected by the metrology tool may include transistor gate widths, via or plug diameters, recessed line widths, or three-dimensional semiconductor bodies, though the embodiment is not so limited.
In one embodiment, the port may provide a communications pathway for receiving process history and pattern parameters of a first substrate, and for transmitting a process flow of a second substrate. Process history may comprise a collection of data such as process recipes, tool and chamber identifications, status, event reporting, in-line parametric data, or defectivity maps, or other information specific to the manufacturing steps used to fabricate a semiconductor device, liquid crystal display, or a microelectromechanical system. The process history can include information for all or any portion of the processes to which each substrate is subjected. A process flow may comprise process tool, process chamber, or recipe information for a semiconductor device, liquid crystal display, or a microelectromechanical system on a substrate to be processed.
A uniformity of critical dimensions of a substrate 390 extracted from the pattern parameter data may be used by the controller 310 to estimate a thermal response. In this embodiment, the controller 310 creates at least one intelligent setpoint for each of the plurality of hotplate segments, described herein. The incoming substrate 390 is then heated according to the intelligent setpoints to reduce critical dimension variation across the substrate 390, profile variation across the substrate 390, or uniformity variation across the substrate 390, or a combination of two or more thereof by controlling an actual temperature of each of the plurality of zones of the hotplate 205 using a corresponding one of the plurality of intelligent setpoints during processing.
Controller 310 may comprise a microprocessor, a memory (e.g., volatile and/or non-volatile memory), and a digital input/output port for transmitting and receiving data. A program stored in the memory may be used to control the aforementioned components of a heat treatment apparatus 300 according to a process recipe. Controller 310 may be configured to analyze the process data, to compare the process data with target process data, and to use the comparison to change a process and/or control the processing system components. Alternatively, the controller 310 may be configured to analyze the process data, to compare the process data with historical process data, and to use the comparison to predict and/or establish an endpoint.
In one embodiment, a cooling device 315 is provided around the hotplate 205. Air or nitrogen gas may be provided to one or more surfaces of the hotplate 205 by cooling device 315. The cooling device 315 can communicate with a gas supply source (not shown) at the upstream. Controller 310 can control the flow rate of gas flowing from the cooling device. In an alternate embodiment, heat treatment apparatus 300 may include a monitoring device (not shown) that, for example, perm its optical monitoring of the substrate 390.
With reference to
Alternately, any of hotplates 205, 405, and 605 may be constructed in the jacket form having at least one hollow and at least one recess. The substrate 390 (
The processing of substrates 390 may involve CD control, profile control, and/or uniformity control within each substrate and/or from substrate to substrate. For example, variations in CD measurements, profile measurements, and/or uniformity measurements may be caused by or compensated for by variations in thermal profile across substrate 390 zones and variations in thermal response from substrate-to-substrate and/or from lot-to-lot. An adaptive real-time CD (ARCD) control system may be used to compensate for these variations to produce consistent and reproducible critical dimensions within each substrate, substrate-to-substrate and/or from lot-to-lot.
A static model (not shown) or a dynamic (e.g., virtual state) thermal model 704 characterizing the thermal response of the system may be created using instrumented substrates 1210 (FIG, 12) and may include the interaction between heater zones of the hotplate 205 and the substrate 390 (
In a static embodiment, a static model may be employed to initially set up the heaters and then run the processes in a non-dynamic model with no real time feedback.
With reference to
With reference to
The intelligent setpoint controller 916 can calculate and provide time varying setpoints (TVS) 932 to the multivariable controller 922. The intelligent setpoint controller 916 and the multivariable controller 922 can comprise hardware and/or software components. The virtual sensor 920 may provide substrate 390 temperatures and/or hotplate temperatures 934 to the multivariable controller 922.
Virtual sensors 920 eliminate the need for instrumented substrate(s) 1210 during production and provide an offset for relatively fixed input variables that may otherwise affect expected outputs such as hotplate temperatures 934. For example, a thermal model 704 and virtual sensors 920 may be created once for the hotplate system 1042., the thermal model 704 may be tuned with a few substrates 390 during initial qualification of the thermal or coating/developing system 100.
In, addition, the thermal model 704 receives control inputs 1162 (U), such as heater power, and disturbance inputs (D) 1156, such as unmeasured variations, and determines regulated outputs (Z) 1158, such as substrate 390 temperatures, and measured outputs (Y) 1160, such as hotplate temperatures. The model structure may be expressed as Z=M1U+M3D and Y=M2U+M4D. Alternately, a different expression for the model structure may be used.
The thermal model 704 tracks the “state” of the system, and relates the inputs 1162 to regulated outputs 1158 and measured outputs 1160 in real-time. For example, U, Y may be measured, and by using the thermal model 704, D may be estimated using Y=M2U+M4Dest and Z may be estimated using Zest=M1U+M3Dest.
Pattern parameter data is incorporated into the thermal model 704 when creating the thermal model 704 to compensate for variability that is expected to be added by downstream processing. The compensation provided by the thermal model 704 is designed to counteract the net variability added by one or more subsequent processes. Multivariable controllers (not shown) may be used to calculate the zone-to-zone interaction during the ramp and stabilization modes. An intelligent setpoint controller of thermal model 704 may be used to parameterize the nominal setpoints, create intelligent setpoints using an efficient optimization method and process data, and select appropriate models and setpoints during run-time.
One step in an intelligent setpoint control (ISC) methodology to construct an intelligent setpoint controller 916 (
Several approaches are available for creating thermal models 704 including, but not limited to, first principles models based on heat transfer, gas flow, and reaction kinetics, and on-line models created with real-time data collected from a processing system, such as a thermal processing system 700.
In a first principles dynamic thermal model for characterizing the intelligent set point controller 916 (
where the parameters are:
kw Substrate thermal conductivity
Vk Volume of kth segment
Ak Area of kth segment
dk Distance between the kth and the (k-1)th segment
Ck Contact area between the kth and the (k-1)th segment
δk Air gap distance between the kth segment and the hotplate
ρ Substrate density
Cp Substrate heat capacity
Ta Ambient temperature
h Heat transfer coefficient to ambient
ka Air gap thermal conductivity
Tp Plate temperature
Tk Substrate temperature
The parameter δk depends on the location of the element and may be specified according to the substrate 390 shape. Similarly, the hotplate 205 is also partitioned into concentric segments and described by a similar mathematical relationship.
In one embodiment for modeling the ISC, thermocouples are assumed to be co-located with the heater 325 in the hotplate 205 and any dynamics (e.g., time constants for thermocouple response) associated with the thermocouples are not included in the model. In effect, the model assumes instantaneous temperature measurements. Alternately, thermocouples are not co-located with the heater in the hotplate 205, and/or any dynamics associated with the thermocouples may be included in the model. Energy may be transferred between the plate and the substrate 390 via an air gap. The air gap for each element depends on the substrate 390 radius of curvature and may be included in the model.
The first principles dynamic thermal model defines a set of n differential equations, which may be expressed in compact form by the equation {dot over (T)}=f(T,Tp,Ta). Here, T is a vector that represents the n substrate 390 element temperatures. Simulations using these differential equations may be used to induce variations in thermal response, and hence thermal dose, across the substrate 390. In an alterative embodiment, the ISC may be described by an on-line thermal model. For example, one method to obtain dynamic thermal models can use real-time data collection. In such real-time models, dynamic thermal models are created based on real-time data collected from a hotplate 205, for example.
One method for collecting substrate 390 temperatures is using an instrumented substrate 1210 as shown in
The on-line thermal model may define a dynamic system with heater powers as inputs and the various temperatures, substrate 390 as well as sensor, as output, and the model may be represented by a set of linear differential equations: {dot over (T)}=f(T,P) where the function f(T,P) is linear. To obtain the closed-loop system, a known controller may be applied around this set of equations to obtain the closed-loop response. This method can provide a high fidelity model of the substrate 390 temperature thermal response. The on-line thermal model may, alternatively, be described by multiple linear models that describe the thermal behavior across a broad temperature range. For this purpose, the substrate 390 temperatures may be measured at multiple temperature ranges, and a model may be created that switches from one temperature range to the next as needed.
Pattern parameter data from a first substrate may be incorporated into either the first principles model or the on-line thermal model, which are described above, for establishing intelligent setpoint control of a second substrate. In addition. substrate bow data may be incorporated into the first principles model. For the first principles model, the gap between the substrate 390 and hotplate 205 for each substrate 390 element may be directly modeled. For example, if rc is defined as the radius of curvature of the substrate 390, then, the substrate 390 subtends an angle
Based on this angle, the air gap at a given radial location may be computed as:
During model development, a first principles model including pattern parameter data from a first substrate and bowing data from a second substrate bowing may be implemented numerically on a suitable microprocessor in a suitable software simulation application, such as Matlab. The software application resides on a suitable electronic computer or microprocessor, which is operated so as to perform the physical performance approximation. However, other numerical methods are contemplated by the present invention.
A method for providing critical dimensions of a pattern on a substrate is illustrated in F 13. In element 1300, a plurality of pattern parameters on a first substrate are measured using a metrology system. In one embodiment, a metrology tool such as a spectroscopic ellipsometry tool, an atomic force microscope, or a scanning electron microscope is used to measure the width of a gate of a transistor on a first area of the substrate 390. This measurement is a first pattern parameter collected by the metrology tool. The metrology tool may also measure the width of a gate of a transistor from a second area of the substrate 390. This measurement is a second pattern parameter collected by the metrology tool, resulting in a plurality of pattern parameters. Additional pattern parameter measurements may be collected to provide a uniformity map of pattern features on a substrate. In another embodiment, measurements collected by the metrology tool may include via or plug diameters, recessed line widths, or three-dimensional semiconductor bodies, though the embodiment is not so limited.
In an alternate embodiment, a metrology tool is used to collect pattern parameters in a microelectromechanical system (MEMS) manufacturing environment. As an example, a metrology tool measures the width of a structure, such as an actuator or beam, on a first area of a substrate. This measurement is a first pattern parameter collected by the metrology tool. The metrology tool also measures the width of a structure on a second area of the substrate. This measurement is the second pattern parameter collected by the metrology tool, resulting in a plurality of pattern parameters. Additional pattern parameter measurements, may also be collected to provide a uniformity map of pattern features on a substrate.
The plurality of pattern parameters measured by the metrology tool is received, as described in element 1310. In one embodiment, the plurality of pattern parameters is transmitted from the metrology tool and received by a lithography tool, comprising a hotplate 205, through a wired connection. In another embodiment, the plurality of pattern parameters is transmitted from the metrology tool and received by the lithography tool, comprising a hotplate 205, through a low-power wireless connection. The plurality of pattern parameters may be communicated point-to-point or through an intermediate module, such as a factory automation system. One part of a factory automation system is an information automation system. An information automation system is associated with the execution of process steps by a manufacturing execution system, communication connections, and monitoring of the process equipment for recipe and process management, and material identification and tracking.
A process history for the first substrate is received from an information automation system, as shown in element 1320. The process history is a collection of data, gathered by the information automation system, comprising data such as process recipes, tool and chamber identifications, status, event reporting, in-line parametric data, defectivity maps, as well as other information specific to the manufacturing steps used to fabricate the semiconductor device, LCD, or MEMS. In one embodiment, the process history comprises tool, chamber, recipe, and event time information for the first substrate. The process history can include coating, development and etching steps for the first substrate.
A thermal model is created by the thermal or coating/developing system 100 based at least in-part on the plurality of pattern parameters of the first substrate, as shown in element 1330. The thermal model may define a dynamic system with heater powers as inputs and various temperatures, substrate 390 as well as sensor, as outputs. A plurality of intelligent setpoints are established as shown in element 1340, using the thermal model, wherein each of the plurality of intelligent setpoints is associated with a corresponding one of a plurality of zones of a hotplate 205. The plurality of zones of a hotplate 205 may be a series of annular ring segments 420, a group of sectors of a circle, or a grid of rectangles of a rectangular hotplate 205. The intelligent setpoints may be static or dynamic in reference to a processing of a second substrate. The intelligent setpoints are derived to compensate for a substrate 390 profile, comprising substrate 390 topography information, substrate 390 layer information, or uniformity data gathered from a single substrate 390 or from a plurality of substrates where a repeated pattern of non-uniformity is detected.
As described in element 1350, a second substrate is positioned proximate to the hotplate 205 for thermal processing. In one embodiment, the second substrate is positioned above the hotplate 205, separated from the hotplate 205 by a thin film of gas such as air, and heated by convective and radiation heat transfer. In another embodiment, the second substrate is positioned directly on the hotplate 205, allowing the second substrate to be heated by conducting heat directly from the plurality of zones of the hotplate 205 to the second substrate. An actual temperature of each of the plurality of zones of the hotplate 205 is controlled using a corresponding one of the plurality of intelligent setpoints, as described in element 1360. In one embodiment, the actual temperature of a first zone of the hotplate 205 is higher th,an one or more remaining zones of the hotplate 205 to control critical dimension variation across the second substrate, profile variation across the second substrate, or uniformity variation across the second substrate, or a combination of two or more thereof, based on a plurality of pattern parameters measured on a first substrate. Such control can result in reduced variations. In another embodiment, the actual temperature of a plurality of zones of the hotplate 205 is higher than one or more remaining zones of the hotplate 205.
A process flow is established, as described in element 1370, for the second substrate based at least in part on the process history of the first substrate. In one embodiment, the process flow for the second substrate comprising tool, chamber, and recipe information is matched to the process history of the first substrate. In this embodiment, the established process flow pre-determines a process path for the second substrate so that the second substrate is processed in the same tools, chambers, and recipes as the process path of the first substrate. As a result, the variation induced by the process path is pre-compensated by the thermal process of the thermal or coating/developing system 100, thereby controlling critical dimension variation across the second substrate, profile variation across the second substrate, or uniformity variation across the second substrate, or a combination of two or more thereof. Such control can result in reduced variations.
The process described above and illustrated in
A plurality of embodiments of a method and apparatus for providing critical dimensions of a pattern on a substrate has been described. The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. This description and the claims following include terms, such as left, right, top, bottom, over, under, upper, lower, first second, etc. that are used for descriptive purposes only and are not to be construed as limiting. For example, terms designating relative vertical position refer to a situation where a device side (or active surface) of a substrate or integrated circuit is the “top” surface of that substrate; the substrate may actually be in any orientation so that a “top” side of a substrate may be lower than the “bottom” side in a standard terrestrial frame of reference and still fall within the meaning of the term “top.” The term “on” as used herein (including in the claims) does not indicate that a first layer “on” a second layer is directly on and in immediate contact with the second layer unless such is specifically stated; there may be a third layer or other structure between the first layer and the second layer on the first layer. The embodiments of a device or article described herein can be manufactured, used, or shipped in a number of positions and orientations.
Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above teaching. Persons skilled in the art will recognize various equivalent combinations and substitutions for various components shown in the Figures. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
Claims
1. A method of providing critical dimensions, comprising:
- measuring a plurality of pattern parameters on a first substrate using a metrology system;
- receiving the plurality of pattern parameters of the first substrate;
- receiving a process history for the first substrate from an information automation system;
- creating a thermal model based at least in part on the plurality of pattern Parameters of the first substrate;
- establishing a plurality of intelligent setpoints using the thermal model, wherein each of the plurality of intelligent setpoints is associated with a corresponding one of a plurality of zones of a heater;
- positioning a second substrate proximate to the heater;
- controlling an actual temperature of each of the plurality of zones of the heater using a corresponding one of the plurality of intelligent setpoints during processing to control critical dimension variation across the second substrate, profile variation across the second substrate, or uniformity variation across the second substrate, or a combination of two or more thereof; and
- establishing a process flow for the second substrate based at least in part on the process history of the first substrate.
2. The method of claim 1, wherein the thermal model is a dynamic thermal model.
3. The method of claim 1, wherein the pattern parameters are critical dimensions of a pattern on the first substrate.
4. The method of claim 3, wherein the pattern on the first substrate comprises circuit features.
5. The method of claim 1, wherein the process history includes at least one of a process tool, process chamber, and process recipe information.
6. The method of claim 5, wherein the process flow for the second substrate matches the process tool, process chamber, and process recipe information of the process history of the first substrate.
7. The method of claim 1, further including:
- modeling a thermal interaction between the zones of the heater; and
- incorporating the model of the thermal interaction into the thermal model of the system.
8. The method of claim 1, further including:
- creating a virtual sensor for estimating a temperature for the substrate; and
- incorporating the virtual sensor into the thermal model of the system.
9. The method of claim 1, further including:
- modeling a thermal interaction between the heater and an ambient environment; and
- incorporating the model for the thermal interaction into the thermal model of the system.
10. The method of claim 1, wherein the controlling is to reduce the critical dimension variation, the profile variation or the uniformity variation.
11. A system, comprising:
- a substrate handling system;
- a heater comprising a plurality of heat treatment zones;
- an interface, for receiving process history and pattern parameters of a first substrate, and for transmitting a process flow of a second substrate; and
- a controller for creating a thermal model and establishing a plurality of intelligent setpoints, based at least in part on the plurality of pattern parameters, and for controlling an actual temperature of each of the plurality of zones of the heater based at least in pad on the plurality of intelligent setpoints.
12. The system of claim 11, wherein the interface receives the process history from an information automation system and transmits a process flow of a second substrate to a information automation system.
13. The system of claim 11, wherein the interface comprises a plurality of communication ports.
14. The system of claim 11, wherein the heater is a hotplate,
15. The method of claim 1, wherein the process history is obtained for coating, developing or etching or any combination of two or more thereof.
16. The method of claim 1, wherein the measuring, the parameter receiving, the process history receiving, the creating and the establishing are repeated at least once.
17. The method of claim 2, wherein the process flow establishing is static.
18. The system of claim 11, wherein the process history is obtained for coating, developing or etching or any combination of two or more thereof.
19. The system of claim 11, wherein the controller repeats the creating and the controlling at least once during setup.
20. The system of claim 11, wherein the controller performs the creating dynamically during setup and statically during manufacturing.
Type: Application
Filed: Mar 13, 2007
Publication Date: Sep 18, 2008
Applicant: TOKYO ELECTRON LIMITED (TOKYO)
Inventor: Howell R. Phelps (Austin, TX)
Application Number: 11/685,570
International Classification: G06F 19/00 (20060101);